Peter Frost's anthropology blog, with special reference to sexual selection and the evolution of skin, hair, and eye pigmentation

Thursday, September 24, 2009

Female face shape and sexual selection

Denise Liberton, an anthropologist at Pennsylvania State University, has been studying variation in human facial features. At an upcoming meeting of the American Society of Human Genetics, she’ll be presenting a comparative study of European and West African facial morphology. The main thrust of her presentation is that the shape of the face has differentiated among human populations in part through a selective force that acts primarily on women—and not on both sexes.

We found that several pairwise distances differed between the sexes. For example, the distance from the brow to nasal bridge was found to be more than 5% larger in females than males. We then tested for an interaction between sex and genetic ancestry by testing for differences in the slopes of the ancestry association between males and females. Although the pattern differed slightly between samples, after Bonferroni correction many correlations were the found to be same in both sexes. However, females in all three samples had many additional significant correlations that were not seen in males, while males had very few correlations that were not found in females. The results of these analyses suggest that selection on females is driving the differentiation in facial features among populations. (Liberton et al., 2009)

What is this selective force that acts mainly on female morphology and carries male morphology along in its wake? I suspect Denise Liberton has sexual selection in mind. If so, this finding would support Darwin’s belief that “the races of man differ from each other and from their nearest allies, in certain characters which are of no service to them in their daily habits of life, and which it is extremely probable would have been modified through sexual selection” (Darwin, 1936 [1888], p. 908).

Darwin was puzzled not only by the considerable physical differences separating humans from apes, but also by the considerable physical differences among human populations (Darwin, 1936 [1888], p. 530-531). He concluded that sexual selection was “the most efficient” cause of this differentiation (Darwin, 1936 [1888], p. 908).

Yet sexual selection usually acts on males in other mammals. The females are the ones who normally do the selecting. This is because they must take time out from the mate market for pregnancy, breastfeeding, and infant care. Meanwhile, the males never really leave the mate market, with the result that too many of them are competing for a limited number of available females.

This mammalian ‘law’ has influenced much writing about sexual selection in humans. According to Naomi Wolf, author of The Beauty Myth, "for women to compete with women through 'beauty' is a reversal of the way in which natural selection affects all other mammals" (Wolf, 1990, p. 3). She points to indigenous peoples in sub-Saharan Africa, Australia, and New Guinea as proof that the original human state was one of males vying for the attention of females.

I could cite other writers, but the gist of their argument is always the same. Authentic human nature is represented today by indigenous tropical peoples. They are what we were. Therefore, human nature is about polygynous males who devote little time and energy to raising their progeny and a lot to seducing the limited number of females. Women are thus the ones who have been sexually selecting.

In this kind of argument, we means ‘people of non-tropical origin,’ particularly those of European descent. Yet clearly this argument is false. We were not them for a long time. Europeans have an evolutionary history going back some 35,000 years on their continent. And this was when and where they evolved their current physical appearance: the shape of their face, the color of their skin, hair, and eyes; the length and form of their head hair. To understand why Europeans look the way they do, we should understand how their environment of sexual selection differed from that of tropical humans.

Ancestral humans were exposed to pressures of sexual selection that varied along a north-south axis. In the tropical zone, women could gather food year-round, thus making the cost of a second wife relatively low. With so many being scooped up, female mates were a limited resource. Too many men had to compete for too few women. The pressure of sexual selection was thus on men, with women being the ones who could pick and choose mates.

This situation reversed as humans moved away from the tropical zone. First, it became costlier for a man to provide for a second wife because women contributed less to the family food supply, the longer winters reducing opportunities for food gathering. Second, male mortality increased relative to female mortality because men had to hunt over longer distances. Together, these two trends resulted in too few men competing for too many women. This was particularly so on continental steppe-tundra, where women had almost no opportunities for food gathering and where men had to hunt wandering herds of herbivores over long distances (Frost, 2006; Frost, 2008).

Because of a geographic accident, i.e., a glacial mass over Scandinavia, it was in Europe during the last ice age (25,000 to 10,000 years ago), specifically on the northern and eastern plains, that continental steppe-tundra reached furthest to the south and covered the most territory during the time of modern humans. And this was when and where Europeans came to look European. They did not change in physical appearance because of climatic adaptation. The cause was a change in the direction and intensity of sexual selection: men were now selecting women, and to a much greater degree than elsewhere.

References

Darwin, C. (1936) [1888]. The Descent of Man and Selection in relation to Sex. reprint of 2nd ed., The Modern Library, New York: Random House.

Softer food is often suggested to have led to changes in facial morphology, however Mammoth "luau-style" 29,000 B.C.

[Jiri] Svoboda, a professor at the University of Brno and director of its Institute of Archaeology, and colleagues recently excavated Pavlov VI, where they found the remains of a female mammoth and one mammoth calf near a 4-foot-wide roasting pit. Arctic fox, wolverine, bear and hare remains were also found, along with a few horse and reindeer bones.

"The meats were cooked luau-style underground. Svoboda said, We found the heating stones still within the pit and around."

The presence of "heating stones" and "boiling pits" clearly indicates that (nice and tender) slow cooked meat was being eaten by 29,000 BC, therefor changes to the shape of jaws and dental crowding ought to have first shown up 14,000 years before they actually did if softer food had anything to do with those changes.

"All of the mandibles presented incisor crowding with a majority of minimal and moderate irregularities, but in seven cases there were extreme irregularities and in two canine impaction was observed. These results are in contrast with the literature where it is reported that malocclusions were rare in prehistoric populations. The findings of this study suggest that crowding may be of a genetic origin and might not be caused by excessive tooth size or changes in environmental factors (masticatory activity)."

This was particularly so on continental steppe-tundra, where women had almost no opportunities for food gathering and where men had to hunt wandering herds of herbivores over long distances

Nutrient-Specific Food Intake and Are Under Recent Selection in EuropeansThey theorise selection for choosing low carb food dates from "the transition to new food sources during the agriculture revolution around 11,000 years ago". Why would there be selection for a farming population not to enjoy eating grains or milk?Surely adaptation would be to the foods which were in plentiful in that time.

More likely those who prefer less carbohydrate rich foods have retained a taste for low carb eating that would have been most adaptive when meat (wandering herds of herbivores) was all there was to eat.

Intense sexual selection of women prevailed among ancestral Europeans during the last ice age. Today, there's very little sexual selection of women anywhere. In fact, an argument can be made that the pressure of sexual selection has now shifted to men. The 'European singularity' has been pretty much abolished.

Tod,

In this case, sexual selection has probably accentuated the femininity of European female faces, and secondarily European male faces, by making certain parts of the face more responsive to circulating estrogen during early development (by increasing the number of estrogen receptors or by favoring receptor variants that are more estrogen-sensitive).

This being said, selection acts on phenotypes, and only indirectly on genotypes. If there is selection for women who look more feminine, it doesn't really matter how this comes about developmentally. There could, for instance, be selection for women who look more feminine for reasons unrelated to their prenatal level of circulating estrogen or the estrogen responsiveness of their body tissues.

Do you think the heavy beard growth in European men could have evolved though selection against the appearance of femininised looks in men?

It seems odd that in their prime reproductive years European males' facial features are concealed. The shape of the jaw is said to be particularly significant for female attractiveness, male jaws may have been shifted to a greater extent than other features. If male jaws ended up 'feminized' then beards could be a quick and simple solution to the new situation.

The purpose of the link, btw, was that Holland argues that Europeans, esp Northern Europeans, have facial gracilization which mimics femininity. Which applies to the argument of male directed sexual selection moving the population phenotype, including through non-estrogen based channels.

As the link shows "facial gracilization" is a trait that is not independent of 2D:4D. In fact gracization of the facial features also raises 2D:4D ratio by reducing testosteronization which is a non-estrogen based channel.

A trait that raises 2D:4D, as facial gracilization does, is most unlikely to have resulted from female choice or competition among men for women. It would defeat the object for a female to select the man with the most delicate features as the trait produces high 2D:4D sons; it reduces reproductive success in men. I don't see how selection of delicate featured men could never become 'runaway' as it is self limiting (unless the low 2D:4D men are not reproducing for some reason). Holland needs to come up with that reason.

Obviously sex hormones are involved in facial masculinization, which includes robusticity. But sex hormones don't mediate all robusticity, which is why e.g. the greater robustness of a homo erectus skull does not imply that erectus had greater amounts of testosterone.

Likewise, brow ridges may become more prominent with masculine development, and Australoid women may have more robust brow bridges than many European men, but this does not imply that Australoid women are more masculinized than European men.

So there are alleles that determine facial structures (and perception of femininity) unrelated to hormonal differentiation, and, e.g. in populations where men have greater influence over sexual selection, we might expect an increase in both hormonally mediated and non-hormonally mediated pathways to gracilization.

Holland didn't argue that Europeans have lower prenatal testosterone or higher digit ratios. He argued that N. Europeans have finer features for reasons entirely unrelated to sex hormones.

"It would defeat the object for a female to select the man with the most delicate features as the trait produces high 2D:4D sons; it reduces reproductive success in men"

JT Manning has found this in some modern populations. But if this had always been the case there would have been directional selection for lower digit ratios, and no more genetically mediated variation in digit ratio. Instead, almost 80% of the current variation is due to genes.

Non-monogamous men wouldn't have had greater reproductive success in the historical ecologies of Northern European.

"the greater robustness of a homo erectus skull does not imply that erectus had greater amounts of testosterone To my way of thinking it does.

Australoid women may have more robust brow bridges than many European men, but this does not imply that Australoid women are more masculinized than European men. In a sense it does.

So there are alleles that determine facial structures (and perception of femininity) unrelated to hormonal differentiation, and , e.g. in populations where men have greater influence over sexual selection, we might expect an increase in both hormonally mediated and non-hormonally mediated pathways to gracilization How can they be unrelated if they increase together ?

Where male choice is predominant (which requires a favorable operational sex ratio) the population has lower fetal testosterone, and therefore less facial masculinity and higher digit ratios-- which is what we find in Europeans. Not in the Baltic and Finland we don't.

Holland didn't argue that Europeans have lower prenatal testosterone or higher digit ratios..He wouldn't cite that fact as it's evidence that Europeans are feminized. He does argue that Finns and Swedes are more masculine because they have low 2D:4D.

He argued that N. Europeans have finer features for reasons entirely unrelated to sex hormones.But he never explains how that could happen without an operational sex ratio favourable to selection of the 'derived' features. He makes his case with some very unsavoury comparisons, your description of the essay ( "aesthetic variation") and link suggests you approve.

JT Manning has found this in some modern populations. But if this had always been the case there would have been directional selection for lower digit ratios, and no more genetically mediated variation in digit ratio. Instead, almost 80% of the current variation is due to genes.It was the selection on women's 2D:4D which altered men's in the same direction - higher. Just like face shape.

Non-monogamous men wouldn't have had greater reproductive success in the historical ecologies of Northern Europe.Monogamous low 2D:4D men would though.

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